1. Technical Field of the Invention
The invention relates to a system for determining differences in aerodynamic loading of the rotor blades of helicopter and like aircraft to aid in providing acceptable rotor tracking and balance during flight.
2. Description of the Prior Art
Systems now used in addressing the problem of providing acceptable "track and balance" during flight measure the effects of blade loading differences by measuring resulting blade displacements rather than providing measurements with respect to the cause of the displacement. In this regard, a pilot recognizes a typical helicopter track and balance problem simply as an unacceptable rotor induced vibration. The "track and balance" problem is caused by imbalances in the rotor system in the mass and/or aerodynamic characteristics of the system, and there is at this time no way to discriminate between the effects of mass imbalance and aerodynamic imbalance. Presently, the existence of aerodynamic imbalance can presently only be demonstrated by nulling vibration of the rotor by providing mass balance and then providing measurement of blade displacements due to changes in aerodynamic imbalance. Nulling of the rotor vibration involves controlling the rotor r.p.m. and changing the mass balance of the rotors until vibration or shaking is no longer experienced. The blade position or displacement is read in space using conventional techniques (e.g., a strobe light). The process of providing "track and balance" is thus a lengthy and iterative one.
Patents of interest in this general field include the following: U.S. Pat. No. 3,938,762 (Murphy); U.S. Pat. No. 4,041,775 (McNamee); U.S. Pat. No. 4,463,453 (Cohen, et al); U.S. Pat. No. 4,513,619 (Widdall); and U.S. Pat. No. 4,524,620 (Wright, et al).
Briefly considering these patents, the Murphy patent discloses a sensing system for detecting a state of imbalance in a helicopter rotor blade. This system makes use of a vibration signal which represents the magnitude and phase of the imbalance. A difference signal is generated which is used for dynamic counterbalancing of the blade so as to eliminate tracking errors. The McNamee patent discloses a vibration meter tuned to resonate at known frequencies for known sources of vibration or the like in aircraft. Multiple sensors are provided for each of three dimensions. The occurrence of a resonance condition in the complex vibration environment is identified and isolated. The Cohen, et al patent discloses the use of dual microphones in an apparatus detecting acoustic energy. The apparatus utilizes a sound absorbing blanket in the vicinity of the microphones to filter out ambient noise. The apparatus is used to detect and identify undesirable vibration sources in aircraft. The Widdall patent discloses dual vibration sensing transducers used in providing an input to a pulse shaping circuit. The output of the latter is in turn fed to a phase comparator, then filtered. The phase shift of the signal is displayed on a voltage meter. The system is used to test balance of a rotatable member, such as in an aircraft. The Wright, et al patent discloses a system for the detection of acoustic emissions from replaceable rotor blades of a helicopter in order to monitor the aging of the blades in flight. The object of the system is to obtain the maximum useful life of the blades before replacement.